A present invention relates to a high-power injection-type semiconductor laser, and more particularly to a fundamental-transverse-mode stripe buried heterostructure laser with an extremely high optical power density emission.
The maximum available optical power of semiconductor lasers has been limited by the catastrophic optical mirror damage (COMD), which is a local destruction of laser mirrors at high optical power density emission. The critical optical power density of the COMD is typically of the order of 10.sup.6 W/cm.sup.2 in continuous wave (CW) operation. If a transverse-mode is limited to a small width of a few microns, in a fundamental mode or lower order modes, the critical optical power of the COMD is several orders of magnitude less than the above value in the CW operation. Even below the critical optical power, the higher the optical power is, the more intense the mirror oxidation becomes, thereby causing gradual degradation under CW operation. Therefore, the COMD has to be controlled to increase the optical power density.
It has been known that the COMD is caused by absorption of a laser light in an active layer near a pair of reflective surfaces, or mirrors. The absorption of the laser light generates heat and reduces the band gap due to a temperature rise, and results in a further increase in the light absorption coefficient in the vicinity of the mirrors. There are two theories of the probable cause of the COMD. The first theory is that a thermal strain or fusion is generated through the above heat cycle when an intense laser light is emitted. The other theory is that destruction, due to lattice oscillation of stimulated Brillouin scattering, is caused by an increase in the absorption coefficient. When the active layer is uniformly injected and excited, the injected carrier density decreases in the vicinity of the mirrors. This is because the surface recombination velocity near the mirrors is larger than that at an inner portion. Therefore, the effective band gap near the mirrors becomes smaller than that of the inner portion, and the section near the mirrors becomes an absorber of the photon energy of the laser light, which is regulated by a high injection carrier density inside. Accordingly, in an ordinary injection-type semiconductor laser, where the active layer reaches the mirrors, the COMD cannot be avoided irrespective of its configuration, or crystal material.
Recently, the inventors of the present invention have attempted to avoid the COMD by making the band gap of the active layer near the mirrors effectively larger than that of the inner portion, by controlling the impurity density, and by making absorption of laser light near the mirrors smaller. However, because the difference in the band gaps could not be made larger, there was a limitation to the impurity density that could be used to avoid the COMD.
On the other hand, various types of laser configurations have been developed to separate an active layer for injection and excitation from a waveguide layer for laser light emission. An article in Applied Physics Letters, Vol. 32, No. 5, pages 311 to 314, shows that an excellent result is obtained in a transverse-mode controlled by adopting a rib structure in a construction in which the thin active layer has a stripe geometry and is adjacent to the wide waveguide layer. However, even with such configuration, it is hard to avoid the COMD. For example, when the stripe width is 5 .mu.m, the COMD arises at the optical power of about 200 mW (nearly equal to 4.times.10.sup.6 W/cm.sup.2) in a pulsed operation.